Why is my ex dating just weeks after our breakup?

how do you handle an ex getting into a new relationship just 4 weeks after ending a 3-year relationship?
she ended things and said she wanted out but at the same time it felt like she wasn’t completely sure. my ego got in the way and i didn’t fight for her or try to keep the relationship alive. then she kept texting me and we ended up having sex for another 2 weeks after the breakup. we also had dogs together and i was still helping pay for them.
then i found out she was going on dates. i got angry sent her a mean text and told her never to contact me again. two weeks later i found out she was dating the guy she had gone on a date with. she had even told me that date didn’t work out so finding out they were together really messed with my head.
now i feel stupid. part of me thinks i pushed her straight into his arms but another part of me knows i was hurt angry and felt betrayed. i didn’t want her back at the time so i know i can’t be angry that she moved on but my ego and emotions made me act in a way i regret.
now she’s with a guy who has money and i’m sitting here missing her and the life we had. i’ve randomly run into her brother and cousin and they both said they don’t think the relationship will last and that i’m better looking but honestly hearing that just makes me feel worse.
i don’t know what to think anymore. my brain is still completely fried. how do i deal with seeing her move on so quickly after 3 years together? any advice or perspective would really help.